Dielectric waveguides have been studied and used for guiding light since the 1920's. Since then optical fibres have been developed and refined to fill an important role in optical telecommunications. A wide variety of semiconductor waveguides are known, fabricated using a variety of semiconductor wafer and optical layer forming technologies. Channel waveguides, photonic wires, and many intricate planar waveguide dielectric structures can be formed on planar surfaces of wafers.
One of the limitations of dielectric waveguides is that desired waveguide properties (index of refraction, dispersion, attenuation, polarization properties, etc.) are limited to those of the fabrication materials. For example, waveguides for microphotonic devices are desired for a wide and growing number of proposed applications. For practical implementation of microphotonic devices, microphotonic waveguides are needed satisfying several constraints, including: low waveguide propagation loss; low polarization dependence; temperature-independent (athermal) operation; and limited mode dispersion. Additionally, it is imperative to design the microphotonic device in a manner that permits efficient coupling of the light into and out of the microphotonic waveguide, that permits miniaturization of the device using reliable design techniques, and to permit microphotonic waveguides with different refractive indices to be fabricated. It is further desirable to provide microphotonic waveguides that can bend and/or criss-cross with negligible loss and crosstalk, to reduce need for vias when constructing optical circuits.
Light can be confined in a dielectric waveguide by index-guiding or by exciting defect states in a periodic lattice. A conventional planar waveguide confines the light in a core, which has a higher refractive index than the surrounding cladding material. There are some problems with miniaturizing this type of waveguide, for example to produce microphonic circuits. It is known that imperfections at the high refractive index contrast boundary between the core and cladding results in light scattering losses that may be critical to many applications. This scattering loss can be minimized by using larger core size ridge waveguides, albeit at the expense of limiting the minimum bend radii of the waveguide to several hundred micrometers, which may not be desirable for leveraging the potential for compact microphotonic structures. For channel waveguides, scattering efficiency can be reduced by using waveguides with a core area that is small enough to delocalize the waveguide mode, such that the field intensity at the core-cladding boundary is diminished or by making use of an inherent interference effect of the scattered radiation (J. H. Schmid, A. Delâge, B. Lamontagne, J. Lapointe, S. Janz, P. Cheben, A. Densmore, P. Waldron, D.-X. Xu, K. P. Yap, “Interference effect in scattering loss of high-index-contrast planar waveguides caused by boundary reflections”, Optics Letters, Vol. 33, No. 13, pp. 1479-1481, 2008.) However, large dependence of propagation loss on waveguide geometry, wavelength and polarization have been reported for silicon wire waveguides. Such dramatic changes of loss in silicon wire waveguides are a significant obstacle in building practical microphotonic circuits. Low loss (−1 dB/cm) to −1.7 dB/cm have been achieved for transverse electric (TE) polarization modes. However, square core cross-sections are intentionally avoided to reduce the vertical sidewall height, resulting in optimal TE performance at typical waveguides dimensions of 500 nm×200 nm. The penalty for this optimized waveguide TE performance is that a confined transverse magnetic (TM) mode may no longer be supported by the waveguide (depending on the cladding material used). Since sidewall imperfections originate from the etching process, propagation loss can be reduced by using an etchless process based on selective oxidation, although such waveguides support only TE polarization and insertion loss is significant. Accordingly, there are problems using conventional index-guided solid core waveguides for microphotonic applications, especially those that require polarization independence. Moreover there is limited flexibility regarding materials that provide adequate attenuation, are readily patterned, and provide desirable index and dispersion.
Subwavelength periodic structures were first used in the late 19th century by Hertz, when studying radio waves, using a fine grid of parallel metal wires used as a polarizer. In the 1940s electromagnetic wave propagation in a medium structured at the subwavelength scale was first studied, with alternate layers of a dielectric and a metal. Although the subwavelength phenomenon has been known and exploited for many years in free-space optics and recently also in plasmonics, little has been reported of the use of subwavelength structures in dielectric optical waveguides. Subwavelength structures have been used in couplers, Bragg Gratings for wavelength selective reflection and filtering, and for waveguide facets. See [P. Cheben et al., A broad-band waveguide grating coupler with a sub-wavelength grating mirror. Photon. Technol. Lett. 18, 13-15 (2006)] and [J. H. Schmid et al., Gradient-index antireflective subwavelength structures for planar waveguide facets. Opt. Lett. 32, 1794-1796 (2007)], for example.
Propagation losses for line-defect waveguides in 2D periodic lattices are comparatively large (from −8.0 dB/cm to −4.1 dB/cm) and such waveguides furthermore exhibit large wavelength and polarization dependent loss. Thus again these waveguides are limited, particularly when broadband or polarization independent applications are required.
Several researchers have investigated high index contrast waveguides consisting of alternating index of refraction lines that extend along a direction of propagation of the light [e.g. Nonlinear Waves in Subwavelength Waveguide Arrays: Evanescent Bands and the “Phoenix Soliton” by Or Peleg et al. Physical Review Letters 102, 163902 (2009)]. Large index contrast materials with a subwavelength pitch are disclosed. Bloch modes including extended and self-localized states propagating with an effective index between 0 and 1. Such waveguides have very large interfaces between the different materials as they extend the length of the waveguide. Accurate forming of such waveguide cores is technologically challenging, even at modestly small scales.
Applicant described, in a chapter in Extreme Photonics & Applications by Hall et al. (2008 on p. 229 & seq.), subwavelength silicon structures consisting of alternating refractive index segments. The principle of subwavelength gratings, including the suppression of diffraction, and macroscopic effective homogeneity of the structure during light propagation, are noted. A one dimensional free-space subwavelength grating is illustrated and the effective medium theory is invoked to approximate the index of refraction for waves along two directions in free space. There is no suggestion that waveguides could be fabricated having low loss or other properties that would make them practical waveguides. The chapter goes on to review the known uses of silicon-on-insulator SWG structures, including uses as antireflective coatings (triangular SWG), and mode adapters (chirped SWG).
Bragg gratings are well known structures that incorporate alternating core regions of differing refractive index. It is known that light of lower and higher frequencies than those matching a pitch of the grating pass through the grating with less reflection loss than the frequencies that match the Bragg condition. These devices cannot operate in a system unless light with frequencies matching the Bragg condition are conveyed to the Bragg grating. For example, a paper by Dai et al., entitled: Ridge-waveguide-based Polarization Insensitive Bragg Grating Refractometer teaches a waveguide Bragg grating sensor, and in particular teaches polarization insensitivity of certain Bragg fibre gratings.
Accordingly, while there are a wide variety of waveguides known in the art, and there are structures for reflection and coupling that include subwavelength structures, there is a need for design flexibility in terms of index of refraction, and polarization and wavelength mode dispersions in waveguides, especially waveguides that can be coupled to with low loss, with a variety of waveguides, and can be miniaturized.